Tests For Positive ions

Flame Test (Positive ions)

• Put a small amount of the compound to be tested in a nichrome loop.
• Then hold the loop in a blue flame of a bunsen burner.
• Then use the colour of the flame to identify the metal ion.

Lithium (Li+) = Crimson

Sodium (Na+) = Yellow

Potassium (K+) = Lilac

Calcium (Ca2+) = Red

Barium (Ba2+) = Green

Sodium Hydroxide

• Aluminium, Calcium and Magnesium ions form white precipates, so when we react it with sodium hydroxide we know that the unknown compound cantains either; Al3+, Ca2+ or Mg2+ ions.
• If we add more sodium hydroxide then the white precipitate formed with aluminium ions dissolve.
• Calcium and Magnesium ions can be distinguished by a flame test. Magnesium ions produce no colour.

We can identify ions in a substances by following this:

Copper ions = Blue precipitate

Iron ions (l) = Green precipitate 

Iron ions (ll) = Brown precipitate

Carbonates (negative ions)

• If we add dilute acid to a cabonate it fizzes and produce carbon dioxide.
• This is a good test to see if an unknon substance is a carbonate.
• In limewater the carbon dioxide reacts with calcium hyroxide. It forms a white precipitate of calcium carbonate which turns the limewater cloudy.

Halides (chloride, bromide and iodide)

• The test shows whether chloride, bromide or iodide are present in the compound.
• First we add dilute nitric acid, then add silver nirtrate solution, this removes any carbonate ions.
• If a precipate forms then a halide is present in the compound.

To indentify what ion is present we need to look at the colour of the precipitate:

Chloride ions = White precipitate

Bromide ions = Cream precipitate

Iodide ions = Pale yellow precipitate

Sulfates (negative ions)

• We can test for sulfate ions by adding dilute hydrochloric acid, followed by barium chloride solution.
• The dilute hydrochloric acid removes carbonate ions .
• A white precipitate tells us sulfate ions are present